Global position systems, such as the American NAVSTAR GPS and Russian GLONASS, are known. The NAVSTAR GPS developed by the U.S. Department of Defense is a satellite-based radio navigation system which transmits information from which extremely accurate navigational calculations can be made in three-dimensional space anywhere on or near the Earth. Three-dimensional velocity can be determined with similar precision. The GPS uses eighteen to twenty-four satellites that may, for example, be evenly dispersed in three, inclined, twelve hour circular orbits chosen to ensure continuous twenty-four hour coverage world-wide. Each satellite uses extremely accurate cesium and rubidium vapor atomic clocks for generating a time base. Each satellite is provided with clock correction and orbit information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band signals. The pair of signals includes an L1 signal at a frequency of 1575.42 MHz and L2 signal at a frequency of 1227.6 MHz. The L1 and L2 signals are bi-phase signals modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigation data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite.
Upon detecting and synchronizing with a PRN code, a receiver decodes the PRN encoded signal to recover the navigation data, including ephemeris data. The ephemeris data is used in conjunction with a set of Keplerian equations to precisely determine the location of each satellite. The receiver measures a phase difference (i.e., time of arrival) of signals from at least four satellites. The time differences are used to solve a matrix of four equations. The result is a precise determination of the location of the receiver may be determined by a precise measurement of the L1 and L2 frequencies. The measure frequencies are used to determine Doppler frequency shifts caused by differences in velocity. The measure differences are used to solve another set of equations to determine the velocity based upon the Doppler phase shift of the received signal. U.S. Pat. Nos. 6,421,000, 5,990,831 and 5,952,968 by McDowell and assigned to the assignee of the present application describe receivers used in positioning systems.
GPS signals are very low in amplitude and are transmitted using a spread-spectrum signal bandwidth centered at 1575.42 and 1227.6 MHz. The GPS signals cover a frequency spread of about 20 MHz. GPS receivers are subject to disruption by jamming signals, which may be transmitted either as narrow band signals or broadband signals. Known GPS receiver systems may reduce the effects of a narrow band jamming by using frequency-selective filters, such as notch filters, to attenuate the jamming signal. However, broad band jamming signals are more difficult to reduce or eliminate (to “null-out”) as the frequency spread of the jamming signals approximates the frequency spread of the GPS signal. However, because the frequency spreading sequence of the GPS signal is encrypted according to a pseudo-random noise code, the jamming signals cannot be precisely synchronized to the GPS signal. This permits the effects of the jamming signal to be reduced by nulling-out the jamming signal. Further, the signal strength of the jamming signal is typically much greater than the signal strength of the GPS signal and allows the jamming signal to be nulled down to the thermal noise floor.
As described in U.S. Pat. Nos. 5,952,968, and 5,990,831, conventional positioning receivers have utilized anti-jamming circuitry or processing to reduce susceptibility to inaccuracies and poor tracking due to jamming signals. Conventional systems can utilize space time adaptive processing (STAP) and space frequency adaptive processing (SFAP) to reduce errors due to jamming signals. However, under certain conditions, STAP and SFAP processing can impart geometrically dependent delays on the received GPS signals. The geometrically dependent delays can manifest themselves as pseudo range errors. Although a beamformer can correct for these delays, conventional beamformer techniques generally require twelve additional equalizing filters in addition to the four beam outputs. Future systems that require GPS signals from more satellites than conventional systems may require at least twenty four additional equalizing filters. Additional equalizing circuits require additional processing power and additional hardware.
Generally, receivers perform an ionospheric correction calculation to adjust GPS calculations for delays associated with the GPS signals penetrating the ionosphere. One known equation for the ionospheric correction is:(ρL2−ρL1)/1−β)where β=(154/120)2 for L2 correction and β=(120/154)2 for L2 correction; pL2=the uncorrected pseudo range for L2 and pL1 is the uncorrected pseudo range for L1.
If uncorrected pseudo range values are used in ionospheric corrections, a significant error can be imparted by the STAP/SFAP processing circuit. For example, anti-jamming induced errors or delays can be quadrupled during ionospheric corrections.
In conventional anti-jamming GPS receiver systems, ionospheric correction measurements are not calculated once anti-jamming processing begins due to the single frequency nature of the anti-jamming process. Current accuracy requirements for such systems do not require ionospheric corrections (iono processing) in the anti-jamming mode. However, the natural progression towards tighter accuracy requirements forces ionospheric correction measurements to be made when anti-jamming is enabled. For example, GPS anti-jamming systems are proposed with a snapshot iono feature in which measurements are made on the current tracking frequency with anti-jamming processing enabled, then on the opposite frequency with the anti-jamming processing enabled. The snapshot iono feature enables ionospheric corrections to be calculated when one or both of the two frequencies (L1 or L2 signals) are jammed, which has the potential to offer improved ionospheric correction performance in a jammed environment.
However, since anti-jamming processing is required to track the signal on at least one of two frequencies, the antenna pattern generated by the anti-jamming algorithm (e.g., the STAP algorithm) has the potential to introduce large delays on one or both frequencies upon which a satellite can be tracked. The potential for large delays is especially pronounced for satellites that do not receive the benefit of beam steering. As a result, the following problems can exist:    (1) In the case of jamming on only one frequency, any bias that exists on the jammed frequency can be scaled by −1.54 for L1 corrections and 2.54 for L2 corrections. This implies that for pseudo range bias of X meters, the ionospheric correction error is −1.54X for L1 and 2.54X for L2 and that the total (regular pseudo range plus ionospheric correction) bias after ionospheric corrections are applied, is either −0.54X or 3.54X depending on frequency if the beamformer corrections have already been applied to the pseudo range. This error can be quite large when jamming is present on L1 only and the receiver is tracking without the assistance of anti-jamming processing on L2.    (2) For the case of jamming on both frequencies, the difference in biases gets scaled into the ionospheric correction. For an L1 bias of X meters and L2 bias of Y meters, an L1 pseudo range error of X+1.54(Y−X) or 1.54Y−0.54X and an L2 pseudo range error of Y+2.54(Y−X) or 3.54Y−2.54X are implied. Depending on the sign of each error, the pseudo range error can be very significant. If all L1 and L2 are jammers are co-located, any bias would likely be similar and the error in the ionospheric correction would essentially cancel. If the L1 and L2 jammers are not co-located and biases exist, the error does not cancel out in the ionospheric corrections.    (3) When the receiver transitions from the tracking frequency to the alternate frequency to make the snapshot iono correction, the antenna may be undergoing dynamics such that the antenna pattern is changing rapidly. The fast change relative to the delay in making the alternative frequency measurements can cause compensation from the beamformer to become stale. The antenna pattern can change due to the jammer dynamics as well.
Accordingly, there is a need to adjust for delays associated with anti-jamming circuitry before ionospheric corrections are made. Further, there is a need for an ionospheric correction technique that utilizes extrapolation to improve ionospheric corrections in anti-jam systems. Further, there is a need for a method of removing STAP and SFAP induced errors for ionospheric corrections without the hardware complexity, and software complexity associated with conventional beamformer techniques. Yet further, there is a need for a system and method of reducing anti-jamming induced errors in positioning systems. Yet further, there is a need to reduce anti-jamming errors without requiring equalizing filters for each GPS signal.